The semiconductor industry utilizes numerous techniques to separate distinct electronic devices, often referred to as die, from the semiconductor wafer upon which devices are fabricated. A common method for such separation is the use of a diamond saw. Methods for reducing the area on the semiconductor wafer required to be allocated for saw streets are much desired to enable greater area utilization of the wafer for useful die, thereby increasing the yield of die per wafer. Laser technology offers such an opportunity to reduce the street dimensions for dicing of semiconductor wafers.
The use of infrared lasers, such as Q-switched 1064 nm Nd:YAG lasers, for laser processing of silicon is well known to those skilled in the art. However, since silicon is a weak absorber at 1064 nm, significant problems have been encountered in laser dicing processes operating at or near this wavelength. The cut quality is typically observed to be marred by redeposition of silicon along the wafer surface and along the walls of the cut.
U.S. Pat. No. 4,541,035 of Carlson et al. and U.S. Pat. No. 4,589,190 of Anthony describe fabrication of features in silicon devices using 1064 nm pulsed output such as from an acousto-optic Q-switched, infrared (IR) Nd:YAG laser integrated into an ESI Model 25 Laser Scribing System. (See also “Diodes Formed by Laser Drilling and Diffusion,” T. R. Anthony, Journal of Applied Physics, Vol. 53, December 1982, pp. 9154–9164). U.S. Pat. No. 4,618,380 of Alcorn et al. also describes a method of fabricating an imaging spectrometer by processing a silicon device with a laser.
In U.S. Pat. No. 5,543,365, Wills et al. describe a laser scribing apparatus for the purpose of forming polysilicon streaks in silicon wafers using 1064 nm pulsed output such as from a Nd:YAG laser with a pulsewidth exceeding 4 ns. Alternatively, they teach that the frequency-doubled wavelength of 532 nm may be employed.
In “Excimer VS Nd:YAG Laser Creation of Silicon Vias for 3D Interconnections” (1992 IEEE/CHMT Int'l Electronics Manufacturing Technology Symposium), Lee et. al. (Lee) report use of Nd:YAG laser wavelengths at 1064 nm and 532 nm to create vias throughout the surface of a silicon wafer for the purpose of enabling production of multichip modules. Lee reports that when laser drilling through holes in silicon wafers at 1064 nm, molten material frequently condensed onto the walls of the holes once an appreciable depth was reached. This apparent redeposition of silicon made the holes unsuitable for further processing. Lee reports employment of a double drilling process at 1064 nm to improve hole quality. Lee describes employing 532 nm frequency-doubled pulsed laser output from a lamp-pumped, Q-switched Nd:YAG in a trepanning process using rotating lenses that are offset with respect to the incoming laser beam to cut 4 mil (approximately 100 micron (μm)) diameter holes in silicon. He reports the processing parameters used as 833 μJ per pulse at a pulse repetition frequency of 3 kHz with a pulsewidth of 70 ns. Redeposition of silicon around the perimeter and along the walls of the laser drilled via was still observed and a chemical etching process was used to clean the holes.
Lee further reports on using an excimer laser at a wavelength of 248 nm to drill holes in silicon. Holes with very smooth sidewalls were reported due to the very high pulse energies employed. He reports using an energy per pulse of 290 mJ at a pulse repetition frequency of 250 Hz and a focused spot size of 5 mils (approximately 125 μm) to drill a hole through a silicon wafer in 30 seconds. He compared drilling time to the 3 seconds required for the holes drilled using his 532 nm Nd:YAG trepanning technique. Lee suggests a method for reducing the drilling time required for silicon holes by a 248 nm excimer laser through use of a projection technique. As those skilled in the art will recognize, such a technique is reliant upon an appropriate aperture mask for each pattern of holes to be formed using such a technique.
In U.S. Pat. No. 5,870,421, Dahm discusses the problem of use of near infrared lasers for the purpose of dicing silicon wafers. He teaches that the primary cause for poor cut quality resulting from redeposition when employing near infrared lasers is use of laser pulsewidths exceeding about 1 ns. Dahm teaches the use of near infrared lasers with short pulsewidths of less than about 1 ns to solve the deep absorption depth of near infrared wavelengths in silicon, stating that such short pulsewidths may produce surface plasmas which can act as highly absorbing layers. Dahm also mentions that near infrared lasers, such as 1064 nm Nd:YAG lasers, are used for high speed applications because of their ability to produce greater power than UV lasers, arguing that UV lasers cannot develop sufficient power to process silicon at high speeds.
In U.S. Pat. No. 5,593,606, Owen et al. describe advantages of employing UV laser systems to generate laser output pulses within advantageous parameters to form vias through at least two layers of multilayer devices. These parameters generally include nonexcimer output pulses having temporal pulse widths of shorter than 100 ns, spot areas with spot diameters of less than 100 μm, and average intensities or irradiances of greater than 100 mW over the spot areas at repetition rates of greater than 200 Hz.
In U.S. Pat. No. 5,841,099, Owen et al. vary UV laser output within similar parameters to those described above to have different power densities while machining different materials. They change the intensity by changing the repetition rate of the laser to change the energy density of the laser spot impinging the workpiece and/or they change the spot size.
In U.S. Pat. No. 5,751,585, Cutler et al. describe a high speed, high accuracy multi-stage positioning system for accurately and rapidly positioning a wide variety of tools, such as a laser beam relative to targets on a workpiece. They employ a multi-rate positioner system which processes workpiece target positioning commands and converts them to commands to slow and fast positioners. These positioners move without necessarily stopping in response to a stream of positioning data. In one embodiment, this technique enables the laser micromachining of a pattern of small features across a large workpiece, thereby allowing increased throughput of laser micromachined parts.